Applied Surface Science

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Contents lists available atScienceDirect

Applied Surface Science

journal homepage:www.elsevier.com/locate/apsusc

Full Length Article

Enhanced performance and stability of ambient-processed CH

₃

NH

₃

PbI

_3-x

(SCN)

_x

planar perovskite solar cells by introducing ammonium salts

Yuzhu Li

^a,1

, Zongbao Zhang

^a,1

, Yang Zhou

^a

, Lai Xie

^a

, Naitao Gao

^a

, Xubing Lu

^a

, Xingsen Gao

^a

, Jinwei Gao

^a

, Lingling Shui

^b

, Sujuan Wu

^a,⁎

, Junming Liu

^c

aInstitute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, PR China

bGuangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Normal University, Guangzhou 510006, PR China

cLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China

A R T I C L E I N F O Keywords:

CH3NH3PbI3-x(SCN)x

Ammonium salt additives Air-process

Photoelectric properties

A B S T R A C T

Metal halide perovskite solar cells have drawn a lot of attention due to their excellent photovoltaic properties.

However, a simple method to prepare perovskitefilms with high quality in ambient air remains a big challenge, which has become an obstacle for the commercialization of PSCs. Here we propose a facile method to prepare efficient CH3NH3PbI3-x(SCN)x-based planar PSCs in ambient conditions and ammonium salts (NH4Cl, NH4SCN) are used to regulate the microstructure of CH3NH3PbI3-x(SCN)xperovskitefilm prepared in ambient air. At the optimal concentration, the devices with NH₄Cl or NH₄SCN additives achieve the champion efficiency of 14.71%

and 16.61% respectively, which are much higher than the 12.97% of the reference device. The stability of the unsealed devices with additives in ambient air has also been significantly improved. The modified devices without any encapsulation still retain about 80% of initial efficiency after 30 days in ambient air. The conductive atomic force microscopy and photoluminescence measurement are used to characterize photoelectric properties of perovskitefilm. The trap-state density and charge recombination of the devices have been investigated. The results suggest that the improved photovoltaic characteristics and stability may be attributed to the improved quality of perovskitefilms, the reduced trap states and the suppressed charge recombination.

1. Introduction

Organic-inorganic perovskite materials are one of the most popular light absorbing materials due to their many advantages, such as wide absorption range, long carrier diffusion length, excellent semi- conducting properties and so on [1–4]. In the last few decades, the research on organic-inorganic hybrid perovskite solar cells (PSCs) has achieved unprecedented development[5–8]. The certified power con- version efficiency (PCE) has reached 25.2%[9]. However, most of the PSCs are fabricated under anhydrous conditions and the instability issue of perovskite material itself in ambient air is still unresolved [10–12]. To deposit the perovskitefilm with high quality by a simple and scalable process is one of the most important challenges for the commercialization of PSCs. To overcome these issues, much work has been done. For example, some researchers have devoted to improving the photoelectric properties of PSCs by interface engineering and additives[13,14]. To prepare more stable devices, some efforts have been done to fabricate PSCs based on the 2-dimensional perovskite materials.

But the PCE of these devices is lower than most of the 3-dimensional PSCs prepared in the glove box[15–18].

Thiocyanate (SCN) anions are often used as pseudo-halide additives to improve the moisture resistance and eﬃciency of PSCs[19–21].

Moreover, several lead sources such as Pb(SCN)2, PbCl2, PbBr2and Pb (Ac)2were used to replace PbI2 to prepare perovskiteﬁlms. Among them, Pb(SCN)2is one of the important candidates. When the Pb(SCN)2

is deposited on the compact TiO2film in the two-step method, it tends to form a densefilm and cannot react completely with CH3NH3I, resulting in the worse performance of CH3NH3PbI3-x(SCN)x PSCs [19,20,22,23]. As it is well-known that the microstructure of the perovskitefilm has a significant impact on the photovoltaic performance of PSCs and tuning the morphology of perovskitefilm can improve the photoelectric properties of PSCs[24]. In order to improve the quality of perovskite film, many methods have been used such as solvent re- processing [25], gas evaporation treatment [26], additive assistance [27]and so on. Among them, the introduction of proper additives in the perovskite precursor is one of the most widely used methods. As

https://doi.org/10.1016/j.apsusc.2020.145790

Received 12 November 2019; Received in revised form 15 January 2020; Accepted 14 February 2020

⁎Corresponding author.

E-mail address:[email protected](S. Wu).

1They contribute equally to this work.

Available online 15 February 2020

T

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reported in our previous work, the introduction of NaSCN and KSCN additives into the perovskite layer can significantly improve the performance and stability of PSC [23]. It is found that the addition of ammonium cation (NH4+) into organic PSCs can regulate and optimize the microstructure of perovskite films, leading to the enhanced performance and stability of the PSC [14,22,23,28–30]. Moreover, the NH4+cation can better passivate the trap states in PSCs compared to the Na⁺or K⁺due to its larger ion radius[31]. On the other hand, the SCN^-anion can also contribute to improve the PCE and stability of PSCs [23,32]. The ammonium chloride (NH4Cl) has also been used as an additive to improve performance of PSCs[33–35]. Furthermore, it is necessary to transfer the preparation process of PSCs from a glove box to air to develop the lower cost photovoltaic technology. To further improve the performance of device and compare the effects of ammonium thiocyanate (NH4SCN) and NH4Cl additions on CH3NH3PbI3-x

(SCN)x-based PSCs fabricated in ambient air, NH4SCN and NH4Cl are used as additives for CH3NH3PbI3-x(SCN)x-based PSCs in this work.

Moreover, the involved mechanism has been systematically investigated.

On this base, our PSCs with the structure of FTO/TiO2/CH3NH3PbI3- x(SCN)x/Spiro-OMeTAD/Ag have been fabricated in an ambient air.

The NH4SCN and NH4Cl are selected as additives to optimize the microstructure of the CH3NH3PbI3-x(SCN)xfilms, respectively. The effects of NH4SCN and NH4Cl additives on the microstructure of perovskite layer and photoelectric properties of devices have been systematically studied. Three types offilms/PSCs have been analyzed in this work: the Referencefilm/PSC that no any additive is included, the NH4SCN- and NH4Cl-film/PSC where the NH4SCN or NH4Cl are added to fabricate the CH3NH3PbI3-x(SCN)xfilms/PSCs. The results show that the as-prepared NH4SCN- and NH4Cl-film do show larger grain size, homogenous morphology and lower trap-state density, resulting in the improved photovoltaic performance and moisture stability. The highest PCE of the reference PSC is 12.97% while the NH4SCN- and NH4Cl-PSC achieve the champion PCE of 16.61% and 14.71%, respectively. Moreover, the moisture stability of NH4Cl-PSC and NH4SCN-PSC in ambient atmo- sphere has also been significantly improved compared to the reference PSC. The improvement of PCE and stability in the NH4SCN- and NH4Cl- PSCs may be due to the optimization of microstructure and the reduced trap-state density in the CH3NH3PbI3-x(SCN)xfilms. Significantly, all PSCs in our work are prepared by two-step method in ambient air and all processes are carried out at a relatively low temperature, which will reduce cost.

2. Experimental methods 2.1. Device fabrication

Fluorine doped tin oxide (FTO, 15Ω/squares, NSG) substrates were etched using Zn powder and diluted hydrochloric acid, then those FTO glass was washed with detergent, acetone, deionized water and ethanol, successively. For TiO2compact layer, the FTO glass was immersed in

titanium tetrachloride aqueous solution at 70 °C for 55 min, then TiO2

ﬁlms were annealed at 200 °C for 30 min in air. The CH3NH3PbI3-x

(SCN)xfilms were deposited on the TiO2compact layer by a two-step sequential spin-coating method. The Pb(SCN)2 solution (500 mg/ml dissolved in dimethylsulfoxide (DMSO, Sigma-Aldrich) wasfirst spin- coated on the TiO2-coated FTO (FTO/TiO2)film at 4000 rpm for 30 s and annealed at 90 °C for 30 min. For the modified devices, different amounts of NH4Cl (Aladdin) or NH4SCN (Aladdin) were introduced into the Pb(SCN)2precursor solutions. Then, the CH3NH3I solution (8 mg/

ml dissolved in 2-propanol) was spin-coated on Pb(SCN)2 layers at 3000 rpm for 60 s and annealed at 80 °C for 20 min to form a CH3NH3PbI3-x(SCN)xlayer. Subsequently, the Spiro-OMeTAD layer was spin-coated on the cooled substrate at 4000 rpm for 30 s. The Spiro- OMeTAD solution can be obtained by dissolving 72 mg Spiro-OMeTAD, 28.6 µL 4-tertbutylpyridine and 17.5 µL Li-TFSI solution (520 mg/mL in acetonitrile) in 1 ml chlorobenzene. Finally, Ag electrode with the thickness of about 100 nm was thermally evaporated on the top of the devices.

2.2. Characterizations

Scanning electron microscopy (SEM, ZEISS ULTRA 55) was used to characterize the morphology of the perovskitefilm. The X-ray diffraction (XRD) patterns were obtained by the X-ray diffraction system (X'Pert PRO, CuK’a radiation). The software of Nano Measurer was used to analyze the grain size distribution of perovskitefilms in the SEM images. The J-V curves of PSCs were recorded under simulated AM 1.5G irradiation (100 mW/cm²) by applying a solar simulator (Newport 91160, AM 1.5G) combined with a source meter (Keithley 2420). The scanning rate for the J-V curve was 0.10 V/s. The external quantum efficiency (EQE) of the perovskite solar cells was conducted using a standard EQE system (Newport 66902). The UV–vis absorption spectra were measured by a SHIMADZU UV-2550 spectrophotometer. The current–voltage (I-V) curves were acquired by Keithley 2420 source meter in dark. The photoluminescence (PL) spectra were carried out with a fluorescence spectrophotometer (HITACHI F-5000) exited at 410 nm. The time-resolved PL spectra (TRPL) were collected using Horiba Instruments Incorporated-Fluorolog-3 excited at 550 nm. The electrochemical impedance spectroscopy (EIS) of the PSCs was obtained at 10 mV amplitude over the frequency ranging from 1 Hz to 1 MHz using a Zahner electrochemical workstation (Zennium, Germany) under the 30 mW/m²white LED light. The surface photo-current was measured with the conductive atomic force microscopy (Asylum Research, Cypher).

3. Results and discussion

The schematic draw of the device structure is presented inFig. 1a.

To investigate the effect of NH4SCN or NH4Cl additives on crystallinity and morphology of CH3NH3PbI3-x(SCN)xfilms, the XRD patterns and top-view SEM images of the three kinds of perovskitefilms (with or

Fig. 1.(a) Schematic illustration of PSC; (b) XRD patterns of perovskiteﬁlms with and without additives deposited on FTO/TiO2.

Y. Li, et al. Applied Surface Science 513 (2020) 145790

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without additives) are provided. The XRD patterns of reference, NH4Cl- and NH4SCN-film are shown inFig. 1b. It can be seen that no new diffraction peaks appear in the NH4Cl- and NH4SCN-film, suggesting that the NH4SCN or NH4Cl additive does not change the crystal structure of perovskitefilm. Moreover, all thefilms exhibit the characteristic peaks at 14.08°, 28.41° and 31.9° which are corresponded to the (1 1 0), (2 2 0) and (3 1 0) crystal planes[22], respectively. It is noted that the full width at half maximum (FWHM) of the (1 1 0) plane in NH4SCN- film (0.365) is narrower than that of the NH4Cl-film (0.371) and referencefilm (0.376), which indicates that the NH4Cl and NH4SCN additives increase the crystallinity of perovskite films. In addition, the intensity of the PbI2peak (12.5°) in NH4Cl-film becomes weaker compared to the reference[22]and there is no PbI2peak in the XRD pattern for NH4SCN-film. These results confirm that the NH4SCN or NH4Cl addition can suppress the formation of PbI2. The enhanced crystallinity of perovskite and the reduced PbI2formation will facilitate the charge separation and extraction [22,36], resulting in the improved photovoltaic performance of devices. Figs. 2a-c show the top-view SEM images of the three perovskitefilms, respectively. The corresponding distribution of grain size is shown inFigs. 2d-f. It can be seen that the NH4SCN-film exhibits the maximum average grain size (407 nm) with respect to those of the NH4Cl-film (366 nm) and reference film (328 nm). The increased grain size will not only decrease the series resistance of devices but also facilitate to reduce charge trapping or recombination[37], resulting in the enhanced photovoltaic parameters.

They will be conﬁrmed by the data of J-V and trap-state density (Dtrap) of the threeﬁlms, which will be discussed in the following.

In order to fabricate the best devices, the concentrations of NH4Cl and NH4SCN solutions have been optimized.Figs. S1, S2(Supporting Information, SI) show the J-V curves and PCE as functions of the additive concentrations, respectively. The corresponding photovoltaic parameters of the NH4Cl- and NH4SCN-PSC are listed inTables S1, S2, respectively. It can be noted that the NH4Cl- and NH4SCN-PSC achieve the highest PCE at concentrations of 1.5 wt% and 3.5 wt%, respectively.

Therefore, they are the optimal concentration for NH4Cl and NH4SCN additives, which are the condition for NH4Cl- and NH4SCN-PSC in all of our other experiments fromFigs. 3–7,S3 and S4.

The J-V curves of devices are demonstrated inFig. 3a. The reference PSC obtains a maximum PCE of 12.97% (a short-circuit current density (Jsc) of 18.52 mA/cm², an open circuit voltage (Voc) of 1.027 V and a ﬁll factor (FF) of 0.68). While the NH4Cl-PSC get a maximum PCE of 14.71% (Jsc: 19.66 mA/cm², Voc: 1.028 V and FF: 0.72), and the

NH4SCN-PSC achieve a maximum PCE of 16.61% (Jsc: 20.78 mA/cm², Voc: 1.046 V and FF: 0.75). The average PCE for reference PSC, NH4Cl- PSC and NH4SCN-PSC are respectively 12.61%, 14.25%, 16.31%, as shown inTable 1. The eﬃciency statistics histograms are presented in Fig. 3b. The distributions of Voc, Jscand FF are demonstrated inFigs. 3c- e, respectively. It is noted that the NH4Cl-PSCs and NH4SCN-PSCs show the increased Jsc, FF and PCE. Compared to the Vocof reference PSC, the NH4SCN-PSC shows a slight higher value and there is no apparent change in NH4Cl-PSC. The increased Voc, Jscand FF may be due to the better microstructure, the reduced Dtrap and suppressed charge recombination[14,38,39]. To better characterize the device performance, the J-V curves of reference PSC, NH4Cl-PSC and NH4SCN-PSC measured under reverse (RS) and forward (FS) scan directions have been provide, as shown inFig. S3. The hysteresis index (HI) has been calculated according to the report[40]. The values of HI for reference-PSC, NH4Cl- PSC and NH4SCN-PSC is 0.185, 0.140 and 0.114, respectively. Ob- viously, the addition of NH4Cl or NH4SCN can help to reduce HI. This may be related to the reduction of defects on the surface or grain boundaries of perovskiteﬁlm[41,42]. To further verify the accuracy of the J-V measurement, the steady-state PCE for the three types of PSCs have been recorded at a constant bias voltage at the maximum power point (MPP) and the results are shown inFig. 3f. They are 0.81, 0.82 and 0.83 V for reference-PSC, NH4Cl- and NH4SCN-PSC, respectively.

The PCE for reference, NH4Cl- and NH4SCN-PSC respectively stabilizes at 12.39%, 13.99% and 15.84% under the standard AM 1.5 G con- tinuous illuminations for 120 s. These values are consistent with the average PCE of J-V measurement, which conﬁrms the accuracy of the J- V results.

The UV–vis absorption spectrum and EQE curves are measured to investigate the reasons for the enhanced Jscin the NH4Cl- and NH4SCN- PSC.Fig. 4a demonstrates the absorption spectra of the threefilms. The NH4Cl- and NH4SCN-films exhibit higher absorption than the reference film in the range of 380 to 600 nm. This can be attributed to the reduced PbI2residues, the improved surface coverage and crystallinity of perovskite layer in the NH4Cl- and NH4SCN-film[22,24]. The EQE and current density curves of the three devices are collected and presented inFig. 4b. Compared with the reference PSC, the NH4Cl- and NH4SCN- PSC show the increased EQE and current density in the range of 380 to 750 nm. The integrated current density calculated from EQE data for reference, the NH4Cl- and NH4SCN-PSC is 17.98, 19.21 and 20.78 mA/

cm², respectively. They are very close to the average values of Jscob- tained from the J-V measurement. Therefore the higher EQE can Fig. 2.Top-view SEM images of CH₃NH₃PbI_3-x(SCN)_xfilms with and without additives: (a) Referencefilm; (b) NH₄Cl-film; (c) NH₄SCN-film. The scale bar is 1 um.

The histogram of the calculated grain size and Gaussianfitting of the statistical data from SEM images: (a) Referencefilm; (b) NH4Cl-film; (c) NH4SCN-film.

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explain the increased Jscin the devices with additives[23,43]. How- ever, the enhanced EQE is not closely related to the light absorption in the range of 600 to 750 nm, especially for NH4Cl-PSC. This may be attributed to the promoted carrier separation and collection, and the suppressed charge recombination[44,45]. It will be further discussed in the following, as shown inFigs. 6 and 7. In addition, the local electrical properties such as photo-current are analyzed by conductive atomic force microscopy (CFM). The CFM can be used to characterize the conductivity and local current distribution of perovskitefilms[46]. The CFM images of the threefilms are shown inFigs. 5a-c, respectively. The histograms of the average photo-current for the threefilms are shown in Fig. 5d. It can be seen that the values of average photo-current increase from 20.2 pA of referencefilm to 27.5 pA of NH4Cl-film and 39.3 pA of NH4SCN-film. Obviously the values of photo-currents have increased after the NH4Cl or NH4SCN addition, suggesting the better electrical properties in NH4Cl- and NH4SCN-film[47,48]. The result is also consistent with the trend of Jscvariation of the three corresponding devices, which can explain the enhanced Jscin the NH4Cl- and NH4SCN- PSC.

The electron Dtrapof perovskitefilms has an important impact on device performance. Thus the Dtrapof the three types of perovskitefilms was investigated. To get the kink point (trap-filled limit voltage, VTFL) exactly, six electron-only devices with the structure of FTO/TiO2/perovskite/PCBM/Ag were fabricated. The average values of VTFLfor the

Fig. 3.(a) J-V curves of Reference, NH4Cl- and NH4SCN-PSC; (b) Eﬃciency statistics histograms of Reference, NH4Cl- and NH4SCN-PSC; (c) Voc, (d) Jsc, (e) FF and (f) PCE as functions of time for PSCs biased at their MPP.

Fig. 4.(a) Absorption spectra, (b) EQE and the integrated current density curves for Reference, NH4Cl- and NH4SCN-PSC,

Fig. 5.CFM images of perovskite films without and with additives: (a) Referencefilm; (b) NH4Cl-film; (c) NH4SCN-film. The scale bar is 2 um. (d) Histogram of average photo-current obtained from the CFM measurement.

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three devices are shown inTable S3, respectively. The I-V curves corresponding to the average VTFLare shown inFigs. 6a-c, respectively. At low bias, the linear relationship of the I-V curve demonstrates the ohmic response, which means the region is the trap-statesﬁlling stage.

When the applied bias is higher than that of VTFL, the current drama- tically increases. This means that the trap-state ﬁlling process has completed[49,50]. The values of Dtrapcan be obtain by theﬂowing Eq.

(1) [49]:

=eD L V εε

TFL 2

trap 2

0 (1)

whereɛis the relative dielectric constant of perovskite material,ɛ0is the vacuum permittivity (ɛ0= 8.854 × 10^-12F/m), L is the thickness of perovskiteﬁlms, and e is the electron elementary charge (e = 1.6 × 10^-

19C). The thicknesses of the three perovskitefilms were measured and presented inFig. S4. The values of VTFLfor the reference, NH4Cl- and NH4SCN-film are 0.67, 0.48 and 0.26 V, respectively. According to the Fig. 6.I-V curves of the electron-only devices displaying the VTFLkink point behavior: (a) Referencefilm; (b) NH4Cl-film; (c) NH4SCN-film. (d) Histogram of the calculated D_trap.

Fig. 7.(a) Nyquist plots of Reference, NH₄Cl- and NH₄SCN-PSC; (b) Fitted R_recand R_trafor Reference, NH₄Cl- and NH₄SCN-PSC; (c) PL and (d) TRPL spectra of Reference, NH₄Cl- and NH₄SCN-ﬁlm; (e) Photo-voltage decay curves of Reference, NH₄Cl- and NH₄SCN-PSC; (f) Normalized PCE of Reference, NH₄Cl- and NH₄SCN- PSC stored in air without any encapsulation.

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equation (1), the calculated Dtrap is 4.29 × 10¹⁶, 2.4 × 10¹⁶ and 1.24 × 10¹⁶cm⁻³ for reference-film, NH4Cl- and NH4SCN-film, respectively. Obviously the values of Dtrapdecrease after the NH4Cl and NH4SCN addition, suggesting that the defect in perovskite layer is ef- fectively passivated by NH4SCN or NH4Cl additive. They will contribute to enhance the photovoltaic parameters of NH4Cl- and NH4SCN-PSC [51]. This is consistent with our J-V measurements, confirming that the lower Dtrapcan help to achieve better photovoltaic performance of PSCs [50].

The PL, EIS and dark current curves were also recorded to illustrate the effect of NH4Cl and NH4SCN additive on the charge recombination in PSCs. EIS data were measured under illumination with 1.0 V bias voltage. Fig. 7(a) shows the representative Nyquist plots of different PSCs and the equivalent circuit used tofit the curves. The solid line in Fig. 7(a) is thefitting results. The transport resistance (Rtra) is asso- ciated with the interfacial charge transfer which corresponds to high- frequency arc; the recombination resistance (Rrec) is related to low- frequency arc[52]. For more accurate results, we replaced the capa- citance (C) with constant phase angle elements (CPE1 and CPE2), which can better clarify the defects at the interface and the spatial non-uni- formity caused by impurities. The specific parameters of thefitting are listed in Table S4. The histograms of Rrec and Rtraare presented in Fig. 7b. It can be seen that the values of Rtradecrease while the values of the Rrecincrease after the NH4Cl and NH4SCN addition, compared to the Reference PSC. The larger Rrec suggest a smaller charge recombination rate and faster charge transfer in PSCs [53], which will contribute to enhance the Voc, Jsc, FF and PCE of devices[14,54]. The smaller Rtra in NH4Cl- and NH4SCN-PSC is beneficial for charge transfer. Fig. 7c shows the PL spectra of the Reference, NH4Cl- and NH4SCN-film, respectively. We can see that the intensity of PL quickly decreased when the NH4Cl or NH4SCN is introduced. The result sug- gests that the NH4Cl and NH4SCN additions can significantly reduce the charge recombination rate of perovskitefilm[31,55], which will contribute to more effective electron extraction, consisting with the results of the Dtrapand EIS[53]. For a deeper understanding of the effects of additives on carrier recombination in devices, the TRPL spectra and the photo-voltage decay curves of thesefilms were recorded.Fig. 7d pre- sents the TRPL decay curves for the reference perovskitefilm, NH4Cl- film and NH4SCN-film. All of these perovskitefilms were deposited on the same substrates of glass/FTO/TiO2. The TRPL decay curves can be fitted by the bi-exponential decay Eq.(2) [56–58]:

= + − + −

y y A x

τ A x

exp( ) exp( τ)

0 1

1 2

2 (2)

whereτ1is the carrier lifetime of the fast process in the perovskite’

surface and crystal boundaries; τ2is the carrier lifetime of the slow process in the perovskites’grains. A1and A2are the relative amplitude of fast and slow processes, respectively. The mean carrier lifetime (τmean) is related to the defect concentration in perovskiteﬁlm. The formula(3) and (4)can be used to calculated theτmean[57,58]:

= +

B A τ

A τ A τ

i i i

1 1 2 2 (3)

= +

τmean B τ1 1 B τ2 2 (4)

The calculated values of τmean for the reference, NH4Cl-film and NH4SCN-film are 181.77, 122.32 and 90.70 ns, respectively. Obviously, the reference film has the largest τmean, suggesting that NH4Cl and NH4SCN additive reduce the trap-states in perovskitefilms. The lower PL intensity and shorter carrier lifetime in NH4SCN-film and NH4Cl-film mean more efficient carrier transport and extraction, which will contribute to enhance the EQE[45]. Fig. 7(e) demonstrates the photo- voltage decay curves of the three devices. It is noted that the NH4Cl- and NH4SCN-PSC show slower voltage decay compared to the Re- ference PSC, confirming that NH4Cl and NH4SCN additives reduce the trap-states of perovskitefilm, in line with the result of I-V, PL, TRPL and EIS measurements. The reduced Dtrapand charge recombination rates can explain the enhanced FF, Vocand Jsc[59,60]. Meanwhile, the stability of the unsealed PSCs in ambient air was also studied.Fig. 7(f) shows the evolution of the normalized PCE for the Reference PSC, NH4Cl- and NH4SCN-PSC. For the Reference PSC, the PCE only main- tains about 60% of the initial efficiency after 30 days. While the NH4Cl- and NH4SCN-PSC still retain about 79% and 84% of the initial efficiency in the same conditions, respectively. The enhanced stability in NH4Cl- and NH4SCN-PSC may be attributed to the improved microstructure and the reduced defects and PbI2residues on the surface or grain boundaries of perovskitefilm[38,55,61].

4. Conclusions

In summary, CH3NH3PbI3-x(SCN)x-based planar PSCs have been fabricated in ambient air. The NH4Cl and NH4SCN additives are used to regulate the microstructure and photoelectric properties of perovskite layer. After the addition of NH4Cl and NH4SCN additives, the PCE of PSCs is enhanced to 14.71% and 16.61% respectively, compared with the 12.97% of the reference device. Moreover, the stability of PSCs has been signiﬁcantly improved. The results show that the NH4Cl and NH4SCN additives can increase the grain size, improve the crystallinity and light absorption, reduce the trap-state density and suppress charge recombination of perovskite ﬁlm, resulting in the improved performance and stability.

CRediT authorship contribution statement

Yuzhu Li:Investigation, Writing - original draft.Zongbao Zhang:

Investigation, Writing - original draft.Yang Zhou:Validation.Lai Xie:

Validation.Naitao Gao:Validation.Xubing Lu:Funding acquisition.

Xingsen Gao:Funding acquisition.Jinwei Gao:Funding acquisition.

Lingling Shui:Funding acquisition. Sujuan Wu: Conceptualization, Project administration.Junming Liu:Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competingﬁnancial interests or personal relationships that could have appeared to inﬂu- ence the work reported in this paper.

Acknowledgements

We acknowledge theﬁnancial support of the National Key R & D Program of China (2016YFB0401502), Science and Technology Program of Guangzhou (No. 2019050001), NSFC-Guangdong Joint Fund (No. U1801256), the MOE International Laboratory for Optical Information Technologies.

Appendix A. Supplementary material

Supplementary data to this article can be found online athttps://

doi.org/10.1016/j.apsusc.2020.145790.

Table 1

Photovoltaic parameters of Reference-, NH₄Cl- and NH₄SCN-PSCs.

Voc(V) Jsc(mA/cm²)^c Jsc(mA/cm²)^d FF PCE (%)

Reference 1.027 18.52 17.98 0.68 12.97^a(12.61)^b

NH4Cl 1.028 19.66 19.21 0.72 14.71^a(14.25)^b

NH4SCN 1.046 21.17 20.78 0.75 16.61^a(16.31)^b

a Best PCE.

b Average PCE from 40 devices.

c Measured J_scvalues from the solar simulator.

dIntegrated Jscvalues from the EQE curves.

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